1. Field of the Invention
The present invention relates to an apparatus and method for canceling a neighbor cell interference in a broadband wireless communication system, and more particularly to an apparatus and method for canceling a neighbor cell interference by using multiple receiving antennas in a broadband wireless communication system.
2. Description of the Related Art
In general, wireless communication systems use a cellular communication system, which divides its service area into a plurality of sub-areas (i.e., cells) and provides communication service in order to overcome the limitations of the service area and subscriber capacity. Furthermore, since the cellular communication system provides communication service according to the divided cells, it is possible to spatially reuse a frequency by allowing two cells sufficiently spaced from each other to use the same frequency band. Accordingly, the cellular communication system can accommodate a sufficient number of subscribers by increasing the number of spatially-distributed channels. However, a wireless communication system having a frequency reuse factor of 1 has an excellent frequency efficiency but has a problem in that reception performance is deteriorated due to interference by a neighbor cell.
For this reason, it is stipulated in the IEEE 802.16 standard for the broadband wireless communication system that a signal of a base station should be modulated by a low level such as the quadrature phase shift keying (QPSK), a low forward error correction rate should be employed, and the repetition process should be allowed to be used a maximum of six times. Nevertheless, on a fading channel, when a conventional mobile station is used, a high outage probability, which indicates a probability of failure in receiving data, becomes higher around a cell boundary, so that the handover performance is deteriorated. Particularly, since an DL-MAP message, which occupies the most important position in the reception performance and handover, is transferred to all base stations from the same position, interference between base stations is great, so it is necessary to develop an interference cancellation method for improving the DL-MAP message reception performance.
Meanwhile, FIG. 1 is a block diagram illustrating the configuration of a system model for a mobile station which has two antennas and is located around a cell boundary.
A mobile station 104 is located within the cell area of a serving base station 100 and receives a target signal from the serving base station 100.
Thereafter, when the mobile station 104 moves to a handoff area, i.e., a cell boundary area, of the serving base station 100 and a neighbor base station 102, the mobile station 104 receives not only a target signal for performing communication with the serving base station 100, but also an interference signal from the neighbor base station 102.
In this case, since the mobile station 104 has multiple receiving antennas, the mobile station 104 receives both of reception signals defined by Equations (1) and (2) below.y1(k)=hS(1)(k)xS(k)+hI(1)(k)xI(k)+n1(k)  (1)y2(k)=hS(2)(k)xS(k)+hI(2)(k)xI(k)+n2(k)  (2)
Herein, yi(k) represents a reception signal of an ith receiving antenna, hS(i)(k) represents a channel frequency response between the serving base station 100 and the ith receiving antenna of the mobile station 104, and hI(i)(k) represents a channel frequency response between the neighbor base station 102 and the ith receiving antenna of the mobile station 104. xS(i)(k) represents a signal transmitted from the serving base station 100, xI(i)(k) represents a signal transmitted from the neighbor base station 102, and n(k) represents an additive white Gaussian noise corresponding to an kth sub-channel.
The signals defined by Equations (1) and (2), which are received by the mobile station 104 through the multiple receiving antennas, can be expressed as a determinant defined by Equation (3) below.
                                                        Y              =                            ⁢                                                [                                                                                                                                          y                            1                                                    ⁡                                                      (                            k                            )                                                                                                                                                                                                                    y                            2                                                    ⁡                                                      (                            k                            )                                                                                                                                ]                                =                                  [                                                                                                                                          h                            S                                                          (                              1                              )                                                                                ⁡                                                      (                            k                            )                                                                                                                                                                            h                            I                                                          (                              1                              )                                                                                ⁡                                                      (                            k                            )                                                                                                                                                                                                                    h                            S                                                          (                              2                              )                                                                                ⁡                                                      (                            k                            )                                                                                                                                                                            h                            I                                                          (                              2                              )                                                                                ⁡                                                      (                            k                            )                                                                                                                                ]                                                                                                                      ⁢                                                                    [                                                                                                                                                      x                              S                                                        ⁡                                                          (                              k                              )                                                                                                                                                                                                                                      x                              I                                                        ⁡                                                          (                              k                              )                                                                                                                                            ]                                    +                                      [                                                                                                                                                      n                              1                                                        ⁡                                                          (                              k                              )                                                                                                                                                                                                                                      n                              2                                                        ⁡                                                          (                              k                              )                                                                                                                                            ]                                                  =                                                                            H                      ′                                        ⁢                                          X                      ′                                                        +                  N                                                                                        (        3        )            
Equation (3) is identical to a typical multi-input multi-output (MIMO) model. That is, since the mobile station 104 simultaneously receives the signals of the serving base station 100 and neighbor base station 102 through the multiple receiving antennas although each of the serving base station 100 and neighbor base station 102 has one transmitting antenna, the mobile station 104 has the same reception model as the MIMO system.
The MIMO system can estimate and detect the target signal component independently of the interference signal by using various MIMO signal detection techniques, such as a linear minimum mean square error (MMSE) detection technique, a linear zero-forcing linear detection technique, a zero-forcing V-BLAST (Vertical Bell-Lab Layered Space Time), an MMSE V-BLAST scheme, etc., in which the linear MMSE detection technique is most proper for cancellation of interference.
Hereinafter, a method of estimating a target signal component based on the linear MMSE detection technique will be described as an example.
A defining equation using the linear MMSE detection technique may be expressed as the following Equation (4).{tilde over (x)}S(k)=<(H′HH′+αI)−1·H′H>i·Y  (4)
Herein, α represents an inverse number of a signal-to-noise ratio (SNR), and I represents a unit matrix having a size of [2×2]. Also, (·)H represents a conjugate-transpose operation for a matrix, and <·>i represents an ith row in a matrix.
As shown in Equation (4), the linear MMSE detection technique includes multiple times of complex-matrix multiplication operations and inverse-matrix operations. Accordingly, the linear MMSE detection technique requires a great number of operations, so that there is a problem in that the hardware becomes more complicated when the linear MMSE detection technique is implemented.